The engine starting apparatus starts an engine by driving a starter motor with electric power supplied from a battery carried on a vehicle and by transmitting rotation of the starter motor to the engine by way of a transmission unit. Here, the value of an electric current passed to the starter motor has a direct influence upon the time to start the engine and therefore, needs to be several of hundreds of amperes in order for the engine to start within a predetermined time.
Since no counter electromotive force is generated by the rotation at the time to start the starter motor, a rush current flows from the battery to the starter motor and as a result, consumption of power of the battery increases steeply and the battery output voltage decreases temporarily. Consequently, at the time to start the engine, it sometimes happens that operation of a control unit constituted by electronic circuits becomes unstable or a microcomputer used for the control unit is reset.
Accordingly, as a method for starting the starter motor, a system has been proposed according to which a decrease in the battery output voltage during drive of the starter motor is suppressed by means of a controller carried on a vehicle and adapted to control the starter motor (see Patent Literature 1).
In Patent Literature 1, drive of the starter motor is controlled by means of the controller of an engine generator system and the starter motor and the controller are connected with each other through a harness. Then, the rush current is limited by PWM (Pulse Width Modulation)—controlling the consumptive power of starter motor, with the aid of a semiconductor switch connected in series between the starter motor and ground, in such a manner that, as the time elapses, the duty value increases from that immediately after starting, thereby suppressing the battery voltage from decreasing.